1. Field of the Invention
The present invention relates generally to a cathode ray tube and more particularly to a colour cathode ray tube with an improved colour selecting electrode.
2. Description of the Prior Art
A conventional colour cathode ray tube is generally constructed as shown in a schematic diagram of FIG. 1, such that a glass envelope 1 is provided, a colour phosphor screen or layer 2 (composed of a group of respective colour phosphor stripes) is formed on the inner surface of a panel of the glass envelope 1 and a colour selecting electrode 3 is located within the envelope 1 opposing to the colour phosphor screen 2. The electron beam 5 emitted from an electron gun 4 located within the neck portion of the envelope 1 is scanned by an electromagnetic deflecting device 6 provided around the neck portion of the envelope 1 in the horizontal and vertical directions and then passed through the colour selecting electrode 3 to and then impinges on a desired colour phosphor stripe of the phosphor screen 2. In the colour selecting electrode 3, as shown in FIGS. 2 and 3, a number of band-shaped grid elements 11 are stretched between one pair of opposing sides of a frame 10 so as to be held so they are parallel to the phosphor stripes. Each of apertures 12 defined between the adjacent band-shaped grid elements 11 arranged with a predetermined pitch forms a beam penetrating aperture. Such colour selecting electrode 3 is called an aperture grill.
In the colour cathode ray tube utilizing such aperture grill 3, when the vibration caused by an external impulse or vibration caused by a built-in speaker in a television receiver set becomes equal to the inherent vibration of the grid elements 11 in frequency, the grid elements 11 are resonated. The vibration of the grid elements 11 causes the electron beam to be incorrectly detected relative to the colour phosphor stripe and hence causes the deterioration of the picture quality. In order to avoid the mislanding of the electron beam, in the prior art, a damper 13 made of a tungsten wire with the diameter in a range from 20 .mu.m to 30 .mu.m is stretched. This damper 13 is used to hold the grid elements 11 and to transmit the vibration of the grid elements 11 in sequence to the adjacent grid elements to thereby dampen or suppress such vibration. Nevertheless, in the prior art having the above structure, since resonance frequencies of the respective grid elements 11 are substantially the same, the damping effect for the vibration of adjacent grid elements 11 is not satisfactory. The adjacent grid elements 11 coupled to one another by the damper 13 are resonated at the same frequency. The vibration of the grid element 11 was merely prevented by the set of grid elements 11 which are substantially separated, so that it was impossible to achieve the sufficient prevention of the vibration. The deterioration of the picture quality caused by the vibration phenomenon of the grid elements 11 becomes more and more noticeable with the high-definition cathode ray tube.